This invention relates to semiconductor photodetectors, and in particular to Avalanche Photodetectors.
Optoelectronic devices are now receiving a great deal of attention for use in telecommunications networks primarily for their huge information handling capacity. An integral part of the network is the semiconductor photodetector which converts incident light to an electrical signal. A standard type of photodetector is the PIN diode which includes an intrinsic semiconductor light-absorbing layer sandwiched between n-type and p-type semiconductor layers. One of the problems associated with such a device is that the relatively thick intrinsic layer (usually 200-400 nm) limits the transit time of electrons and holes generated by the incident light and therefore limits the speed of the device. Recently, it has been proposed to reduce the intrinsic layer thickness and sandwich it between two waveguide layers in order to increase the absorption length. Such a device still suffers from transit time limitations, since the intrinsic region, which is defined as the region including an electric field when a bias is supplied to the device, includes the thickness of the waveguide layers. It has also been suggested that light could be coupled to the intrinsic region of a standard PIN device by means of a waveguide.
Another type of photodetector is the waveguide avalanche photodetector. This type of device usually includes, as part of the intrinsic layer, an undoped absorbing and waveguide layer, a doped charge layer, and an undoped multiplication layer. Again, such devices have a fairly thick intrinsic layer, usually of the order of 700 to 1000 nm, which limits transit times.
It is desirable, therefore, to provide a photodetector which exhibits fast transit times so that the device can be operated at high speeds.
The invention in one aspect is a semiconductor avalanche photodetector which includes an essentially undoped multiplication layer, a thin, substantially undoped light absorbing layer, and a doped waveguide layer which is separate from the light absorbing layer and is capable of coupling incident light into the light absorbing layer. In other aspects the invention is a method of fabricating an avalanche photodetector and a network and receiver including an avalanche photodetector.